How to create Onyx material in Blender Cycles?
I'm not experienced with cycles but share some ideas on how to approach this. You could start with a Voronoi texture to get the cellular look:


Then add a color ramp with colors occuring in the image. 

The cells in your image have different sizes this means whe need to mix two Voronoi textures which different scales. The borders of the cells are not as smooth as in the Voronoi texture here some noise could help.



the upper part of the nodes (including noise) could look like:

the lower part (overlay) something like this:



Combined by mix shader: